1. Field of the Invention
The present invention relates to a rotary type driving device employing an electromechanical transducer or an appratus provided with such a device.
2. Description of the Prior Art
Electromagnetic motors have been widely used as a rotary type driving device, and lately a compact, high-speed rotary type has been widely used. On the other hand, a variety of apparatuses employing this kind of driving device are required to achieve accurate position control and high resolution, i.e., accurate control of the angle of rotation, and therefore, the apparatuses have been conventionally provided by combining an electromagnetic motor with a reduction mechanism.
However, when using the reduction mechanism, each apparatus increases in size, and it has been difficult, in practicality, to accurately control the angle of rotation due to the backlash of the gears constituting the reduction mechanism and the like.
In order to solve this problem, the applicant has proposed a linearly advancing type driving device employing an electromechanical transducer as shown in FIGS. 13 and 14 as a driving device which is compact and able to achieve position control with high resolution, i.e., accurately. Further, for the purpose of improving its versatility, the applicant has proposed a rotary type driving device employing an electromechanical transducer as shown in FIGS. 15 and 16 (refer to Japanese Laid-open Patent Publication No. HEI 6-261559). The driving device employing such an electromechanical transducer is constructed so that an extension/contraction displacement is generated in the electromechanical transducer, the extension/contraction displacement is transmitted to a driving member and a driven member is moved via a moving member frictionally coupled with the driving member.
Describing this construction in brief, FIGS. 13 and 14 show an example of the linearly advancing type driving device, where FIG. 13 is a perspective view showing a disassembled state, and FIG. 14 is a perspective view showing an assembled state. The linearly advancing type driving device 100 is constructed of a frame 101, support blocks 103 and 104 on the frame 101, a driving shaft 106, a piezoelectric element 105, a slider 102 and so forth. The driving shaft 106 is supported movably in the axial direction by a support block 103a and the support block 104. One end of the piezoelectric element 105 is fixed by bonding to the support block 103, and the other end is fixed by bonding to one end of the driving shaft 106. The driving shaft 106 is supported displaceably in the axial direction (in the direction of arrow "a" and in the direction opposite to it) when a displacement is generated in the direction of thickness of the piezoelectric element 105.
The driving shaft 106 penetrates the slider 102 in the transverse direction, and an opening 102a is formed in an upper portion where the driving shaft 106 penetrates, so that the upper half portion of the driving shaft 106 is exposed. Further, a pad 108 which abuts against the upper half portion of the driving shaft 106 is fitted into this opening 102a, and the pad 108 has its upper portion provided with a projection 108a, where the projection 108a of the pad 108 is depressed by a leaf spring 109, so that a downward urging force F is applied to the pad 108 which abuts against the driving shaft 106.
With the above arrangement, the slider 102 including the pad 108 and the driving shaft 106 are put in pressure contact with each other by the downward urging force F of the leaf spring 109, thereby achieving a frictional coupling.
The operation will be described next. First, a saw-tooth wave drive pulse having a gradual rising portion and a steep falling portion as shown in (a) of FIG. 17 is applied to the piezoelectric element 105, the piezoelectric element 105 is displaced as gradually extended in the direction of thickness in the gradual rising portion of the drive pulse, and the driving shaft 106 coupled with the piezoelectric element 105 is also gradually displaced in the positive direction (in the direction of arrow "a"). In this stage, the slider 102 frictionally coupled with the driving shaft 106 moves in the positive direction together with the driving shaft 106 by a frictional coupling force.
In the steep falling portion of the drive pulse, the piezoelectric element 105 is displaced as it is rapidly contracted in the direction of thickness, and the driving shaft 106 coupled with the piezoelectric element 105 is also rapidly displaced in the negative direction (the direction opposite to the direction of arrow "a"). In this stage, the slider 102 frictionally coupled with the driving shaft 106 substantially stays in the position as a consequence of the inertia force conquest over the frictional coupling force. By continuously applying the aforementioned drive pulse to the piezoelectric element 105, the slider 102 can be moved continuously in the positive direction.
For the movement of the slider 102 in the direction opposite to the aforementioned direction (in the direction opposite to the direction of arrow "a"), the movement can be achieved by changing the waveform of the saw-tooth wave drive pulse applied to the piezoelectric element 105 and applying a drive pulse having a steep rising portion and a gradual falling portion as shown in (b) of FIG. 17.
FIGS. 15 and 16 show an example of the rotary type driving device, where FIG. 15 is a perspective view of it, and FIG. 16 is a sectional view of it. Around a shaft 202 provided on a fixing member 201 are arranged a thrust bearing 204, a driving member 205, a friction disk 206, a rotor 207, a thrust bearing 208 and a spring 209 in this order. A piezoelectric element 215 is placed between a surface 201b of a receiving portion 201a of the fixing member 201 and a surface 205b of a cutaway portion 205a of the driving member 205 and it is fixed by bonding to the surfaces. By adjusting the tightening amount of a nut 210 meshed with a threaded portion 211 of the shaft 202, a frictional coupling force among the driving member 205, the friction disk 206 and the rotor 207 is adjusted.
When a saw-tooth wave drive pulse is applied to the piezoelectric element 215, the rotor 207 rotates via the driving member 205 and the friction disk 206 with respect to the fixing member 201 in the case of a gradual extension displacement of the piezoelectric element 215 similar to the case of the linearly advancing type driving device set forth. In the case of a rapid contraction displacement of the piezoelectric element 215, the driving member 205 rotates in the reverse direction, whereas the rotor 207 stays in the position. By continuously applying a drive pulse, the rotor 207 can be rotated in a specified direction.
The driving device employing the aforementioned electromechanical transducer, the driving member of which has a relatively large size, and is therefore required to be further reduced in weight. Furthermore, it is required to use a frequency higher than the audio frequency that can be heard by the human ear as the drive frequency of the electromechanical transducing element for the prevention of an unpleasant sensation to the human ear. However, for the purpose of efficiently driving the electromechanical transducer at such a high frequency, a further device has been required.